In the field of marine resource development, an underwater device such as a remotely operated vehicle (ROV) and an autonomous underwater vehicle (AUV) is used. In an underwater device to be operated in the deep sea, a hydraulic manipulator driven by a hydraulic cylinder, for example, disclosed in Japanese Patent Publication No. H06-039873, is used in order to deal with sea pressure in the deep sea. In a case where an electric manipulator driven by an electric motor is used in the sea, a pressure equalizing structure is employed in which the electric motor is accommodated and oil-immersed in a container.